Comcast Corp., the country's largest provider of high-speed Internet access, has begun blocking a channel frequently exploited by spammers to send out large volumes of e-mail, a move that many technologists say was long overdue and should be matched by other service providers.

On Monday, the company began targeting certain computers on its network of 5.7 million subscribers that appeared to be sending out large volumes of unsolicited e-mail. Spokeswoman Jeanne Russo said that in those cases, it is blocking what is known as port 25, a gateway used by computers to send e-mail to the Internet.

The result, she said, was a 20 percent reduction in spam.

"We're taking a precision approach . . . against the top talkers of the day," Russo said, referring to the computers being blocked.

The move is the latest in a continuing technological arms race between spammers and Internet companies, which have yet to see lawsuits or federal and state laws make a dent in the volume of unwanted e-mail.

For years, anti-spam activists have been pressuring Internet providers to block port 25 for all users, because it allows e-mail to be sent directly to the Internet without passing through computers operated by the service provider.

E-mail from most residential consumers is processed by their Internet providers' computers, which increasingly have been fortified with filters and other technologies to limit spam and viruses. For these users, blocking port 25 has no impact.

But Comcast and several other Internet providers let many home and small businesses use their own computers, known as servers, to process e-mail. Such customers use their online providers' networks but send e-mail directly to the Internet.

Comcast, with its large number of customers, has drawn the particular ire of the anti-spam community for not addressing the problem sooner. At a recent anti-spam forum, one of its engineers acknowledged that the company had huge numbers of spammers abusing its network.

The issue has become more acute as spammers have gotten more sophisticated. Early on, they would sign up for an Internet account and start sending unsolicited e-mail. When the Internet provider discovered them, they would close up shop and start another account under another name.

Recently, spammers have infected tens of thousands of machines with malicious software code, turning them into "zombies" that operate as mail servers and launching pads for spam.

Legitimate owners of these machines usually don't know their computers have been commandeered. More than 40 percent of all spam now comes from zombie machines, according to some industry estimates.

Russo said Comcast is not blocking port 25 for all its users because it does not want to remove the option for legitimate customers who process their own e-mail. So the company is monitoring traffic and picking out machines that look suspicious.

Richard P. Wong, general manager for messaging at Openwave Systems Inc., said Comcast's efforts are fine as far as they go. But he said his company, which provides software for network operators, recommends that port 25 be universally shut off.

"These open relays will kill the Internet with spam unless they are blocked," said Wong, who also helps lead the Messaging Anti-Abuse Working Group, a coalition of network operators fighting spam. Wong said blocking port 25 is recommended by the group's guidelines of best practices.

Wong said he estimates that about one-third of all Internet service providers block port 25, while another one-third are now considering doing so.

The issue can be especially difficult for small Internet service providers, who don't want to lose customers that want to process their own e-mail. But Wong said forcing customers to have an Internet service provider or other third-party company process e-mail is inexpensive and would pay large dividends for the Internet as a whole.

Large Internet providers vary in their approaches. America Online Inc. and Earthlink Inc. require that all residential e-mail be run through their own servers. Businesses can open accounts and process their own e-mail after being vetted. Verizon Communications Inc. also allows business customers to process their own mail.

George Webb, a group manager of Microsoft Corp.'s anti-spam unit, said getting more aggressive on blocking port 25 "can have a large impact in a short amount of time." He said the company's MSN network is reliant on cable or phone-line partners to provide its broadband service, and Microsoft is "working with them" on the problem.

Webb said he thinks port 25 should be blocked by default, and customers should be required to apply for an exception.